Injection molding assemblies, for the manufacture of large parts generally require very high flow rates of melt to fill the mold quickly. Hotrunner mold nozzles have been used in many applications of this type, and typically have a valve gated nozzle to control the injection of melt from a plasticizer unit and into the mold. Various gate valves are known in the prior art and are often hydraulically or pneumatically operated to move a valve stem axially between a position in which the flow of melt is permitted and a position in which the flow of melt is prevented at a gate orifice proximate or integral with an injection apparatus. The gate valves themselves generally include a piston-cylinder arrangement to move the valve stem.
Due in part to the high operating temperatures within the mold (often times a melt temperature between 300-400 degrees Celsius and a mold temperature of 100-200 degrees Celsius), the piston and cylinder mechanisms are located away from the melt channel, which is heated and has high temperature melt flowing therethrough. Having the piston and cylinder exposed to the high temperatures typically required for melt flow can lead to the piston sealing ring to fail. Thus, a distance must be kept between the cylinder-piston mechanism and the injection apparatus, or otherwise thermal insulation of some sort is required. As will be appreciated by one skilled in the art, having to locate the cylinder and piston at a distance from the injection apparatus results in a larger than ideal mold arrangement. Furthermore, the valve stem becomes more prone to failure and must itself be made larger than would otherwise be preferred.
Various other valve gates are known in the prior art that attempt to improve the sealing of the piston-cylinder arrangement. These include U.S. Pat. No. 4,213,751 issued Jul. 22, 1980 to Fernandez, U.S. Pat. No. 7,467,941 issued Dec. 23, 2008 to Jenko, U.S. Pat. No. 6,555,044 issued Apr. 29, 2003 to Jenko and U.S. Pat. No. 5,635,227 issued Jun. 3, 1997 to Whisenhunt et al.
All of the solutions in these prior art patents show the use of seals variably located within the piston-cylinder arrangement. Most of the seals must be located a distance from the injection apparatus and from the melt channel in the melt distribution manifold due to the high operating parameters at play, and in addition, the piston-cylinder chamber must often be insulated from this heat or even cooled. As is widely known, in the presence of excessive heat, the piston seals can fail.
Another solution that has been attempted in the prior art in order to keep the piston, cylinder and seal in close proximity to the heated melt channel is to use a high temperature sealing device, such as a cast iron piston ring to be used. Many attempts at using cast iron sealing rings have had an unacceptable high rate of leakage. Typically, compressed air has passed from one side of the seal to the other because the ring is split to permit assembly. One example of the use of a cast iron sealing ring that attempts to solve this prior art problem is shown in U.S. Pat. No. 4,832,593 issued May 23, 1989 to Brown. The background section of Brown also describes other attempted solutions to this problem, and problems associated therewith.
All of these prior art solutions also make maintenance fairly difficult as access to the seals for inspection and/or replacement requires significant disassembly of the gate valve and any portions of the manifold holding the gate valve. The Jenko '044 patent attempts to address some of the difficulties in maintenance by providing a removable backplate (column 6, lines 39-42), but still requires that the cylinder be removed, and access be provided to the valve. This patent also addresses other ways of reducing the operating temperature within the cylinder such that less costly seals can be used over a longer life span.
Some prior art solutions also add cooling channels to keep the cylinders cooled below their uncooled operating temperatures. These cooling channels add a degree of complexity to the mold arrangement and increase the manufacturing costs required. In addition, these cooling channels are not failsafe, and mineral deposits in the cooling channel can result in insufficient cooling. There is also an operational issue where the cooling system is not activated by an operator. For example, when injecting molding PEEK, a mold temperature of about 180 degrees Celsius is required and a melt temperature of about 400 degrees Celsius. Typical elastomeric seals, such as Viton™ seals will shrink and fail after repeated exposure to these temperatures. Furthermore, lubricants used with such seals degrade as well. Typical high volume molds run in the order of 4-5 million injection cycles per year. In this environment, the seals become the Achilles heel of the mold and degradation or failure of seals or lubrication results in significant downtime.
Some prior art valve stem cylinders show cylinder integrated or attached cooling channels and cooling fittings with pipes to keep the cylinder seals and the lubricant below a degradation temperature and cold enough to ensure an operating life over month or years. However even with coolant the seals will age and wear out and the lubricant will dissipate and causing preventative maintenance issues or failure. Another problem arises when the coolant (chilled water) clogs the cooling bores with rust, contamination, calcium or other mineral deposition (hard water) or the cooling is not getting activated due to mold operator failure. For example the industry uses flouroelastomer seals that offer a maximum operating temperature of 200 degrees Celsius over a short time period. Most hot runner manifold operating conditions exceed the 200 degrees Celsius mark of up to 400 deg Celsius manifold temperature. A traditional valve stem cylinder assembly will malfunction due to the failure of elastomer seals if not cooled.
There is therefore a need in the art for a gate valve for use in an injection molding apparatus that is less sensitive to operating temperature than the prior art solution, or does not require the cylinder and piston to be located distant from the injection apparatus, and/or improves upon the maintenance requirements of prior art solutions.